An apparatus for this description, which has come into commerce under the designation "Plastograph", comprises a housing with two mutually parallel and intersecting cylindrical bores accommodating respective counterrotating plasticizing screws in intermeshing relationship. These screws are designed to simulate the conditions existing in a thin-screw extruder but are to retain an introduced plastic composition in order to enable a continuous monitoring of certain parameters--especially temperature, pressure and viscosity--during mastication and plastification. Thus, whereas the actual extruder continuously ejects part of the plastic mass circulating therein, e.g. under a discharge pressure of about 300 Kg/cm.sup.2, the testing apparatus referred to has no outlet. Instead, each screw is provided with two oppositely pitched thread sections which are axially separated by an intervening gap toward which the mass to be tested is being propelled from both ends, the two gaps being aligned with each other to define a region in which the mass remains more or less stationary. While this enables a measurement of the aforementioned parameters over an extended period, it does not effectively reproduce the conditions prevailing in an actual extruder.